1. Field of the Invention
The present invention relates to a fuel cell stack. More particularly, the invention relates to a fuel cell stack suitable for use with a liquid fuel circulated therein.
2. Description of the Related Art
A fuel cell is a device for generating electrical energy from hydrogen and an oxidant and is capable of providing high power generation efficiency. One of the main features of a fuel cell is that electrical power is generated through direct power generation, in contrast to conventional power generation methods in which electrical power is generated via thermal or kinetic energy processes. Therefore, in a fuel cell, high power generation efficiency can be expected even in small-scale systems. Furthermore, a fuel cell is considered to be highly environmentally friendly as it emits less nitrogen compounds and the like and generates less noise and vibration than conventional power generation methods. As mentioned above, a fuel cell can effectively utilize the chemical energy contained in fuel and has environmentally friendly characteristics. Therefore, the fuel cell is expected to serve as an energy supply system of the 21st century and has received attention as a promising new power generation system for use in various applications from small to large scale power generation systems, space devices, automobiles, and portable devices. Thus, the technological development of the fuel cell for practical use is well under way.
In recent years, a direct methanol fuel cell (DMFC) has received particular attention as a promising type of fuel cell. In a DMFC, methanol serving as a fuel is supplied directly to the anode without modification to obtain electrical power through the electrochemical reaction of the methanol with oxygen. In comparison to hydrogen, methanol has higher energy per unit volume and a lower risk of explosion or the like and is also suitable for storage. Thus, a DMFC is expected to be employed as a power source for automobiles, portable devices, and the like.
Japanese Patent Laid-Open Publication No. 2005-108850 discloses a fuel cell stack which is composed of a plurality of stacked cells sandwiched between a pair of end plates with collectors and the like interposed therebetween. In the fuel cell stack disclosed in Japanese Patent Laid-Open Publication No. 2005-108850, an inlet of a manifold for distributing liquid fuel to each of the cells and an outlet of a manifold for collecting unreacted liquid fuel discharged from the cells are provided in different end plates.
As liquid fuel passes inside a fuel cell stack, the potential thereof changes. Therefore, a potential difference is generated between the liquid fuel to be supplied to the fuel cell stack and the discharge fuel discharged from the fuel cell stack. Hence, when the discharge fuel discharged from the fuel cell stack is circulated and is resupplied to the fuel cell stack, a voltage is applied to a metal portion such as a pipe, a tank, a pump, or a heat exchanger contacting the liquid fuel. In this state, when the liquid fuel or a by-product such as formic acid contacts the metal portion, corrosion of the metal portion ensues. Furthermore, when metal ions produced by the corrosion pass through the pipe and enter the fuel cell stack, the metal ions are adsorbed onto the electrolyte membrane and cause the ionic conductivity of such to deteriorate. Thus, the performance of the fuel cell stack is reduced.